Bonding wires used as materials for mounting semiconductors, including a gold wire for semiconductor element connection, are materials for connecting a semiconductor chip with external metal terminals. At present, most of such bonding wires are made using a material mainly comprised of gold. The main reason for this is that a so-called “ball bonding” technique of high throughput and high productivity is used for connecting semiconductor devices with external terminals. “Ball bonding” is a method comprising melting one end of a bonding wire to form a free air ball (FAB), pressing the FAB against one electrode to bond with it, and press-bonding the side surface of the bonding wire to another electrode as it is. Hereinafter, the ball bonding will be referred to as “first bonding”, and the bonding for press-bonding the side of the wire to an electrode is referred to as “second bonding” The reason why gold is used so much is that surface oxidation of free air balls or bonding wires is less likely to cause deterioration in bondability of the first and second bondings, and thus the wire bonding in the atmosphere is easy to carry out.
Although strength of bonding wires is increased by work hardening at the time of drawing process, sufficient mechanical strength is not obtained with pure gold, and thus trace amounts of different elements are added thereto.
With recent progress in the downsizing of semiconductor mounting sizes, the sizes of electrode pads have become smaller, and the pitches thereof also have become narrower. As a result, the diameter of the bonding wire also needs to be small, so that a bonding wire of a 15 μm diameter has come into use. The downsizing of the wire diameter will cause the bonding wire to be broken during the manufacture thereof due to the strength thereof being insufficient at the time of drawing, or will make maintenance of a loop thereof difficult at the time of mounting process, such as wire bonding, resin encapsulation, etc. Accordingly, the strength of the bonding wire needs to be improved.
It is important to improve not only the strength of the bonding wire but also the elastic modulus thereof in order to maintain the loop at the time of the mounting process, such as wire bonding, resin encapsulation, etc. Specifically, the elastic modulus of the bonding wire in a longitudinal direction (hereafter Young's modulus) needs to be improved to prevent the occurrence of wire sweep at the time of resin encapsulation.
With the downsizing of semiconductor mounting sizes, the bonding wire is required to have improved bondability as a characteristic thereof. Additive elements generally used for the bonding wire, however, are easily oxidized at the time of formation of FABs. Thus, adding large amounts of the additive elements to the bonding wire will deteriorate the bondability of the first bonding. Moreover, the second bonding is also affected by the additive elements. In general, a better bondability can be achieved in the second bonding by adding smaller amounts of the additive elements as the less surface oxidization of the bonding wire is resulted in that case.
Recently, roundness of a press-bonded ball formed by press-bonding a FAB onto a pad has become an issue in connection with the first bonding. If pad pitches of electrodes on semiconductor chips become narrow with the downsizing of semiconductor chips, then the risk of occurrence of short-circuit caused by the press-bonded balls running over the adjacent edges of pads is increased in the event that shapes of the press-bonded balls in the first bonding get out of round. What is meant by the wording “out of round” herein is certain shapes of a press-bonded ball, which, for example, appear irregular like petals as illustrated in (1) of FIG. 1, or appear ellipse due to anisotropic deformation of the press-bonded ball between the direction of ultrasonic waves and the direction perpendicular thereto under the influence of the ultrasonic waves to assist in bonding at the time of wire bonding.
Additive elements for improving the roundness of press-bonded balls are disclosed in a number of patent publications. For example, 0.05-1% by mass of Cu, Pb, Li, and Ti in Patent Document 1; 0.00001-0.0002% by mass of Ti in Patent Document 2; 0.1-0.8% by mass of Pt in Patent Document 3; and 0.0001-0.01% by mass of Y in Patent Document 4 are disclosed as the additive elements effective for improving the roundness of press-bonded balls.
As for the additive elements effective for improving the roundness of press-bonded balls, neither any quantity thereof nor any particular element is to be specified, and thus, it is considered that they vary depending on how they are combined with other elements.
Bonding wires containing a high content of Ca or rare earth elements used for those of high strength and high-Young's modulus to meet the downsizing of semiconductor chips have a tendency for the roundness of the resultant press-bonded balls to deteriorate. Thus, additive elements which may coexist with Ca or rare earth elements and improve the bondability of FABs while maintaining mechanical characteristic are required.    Patent Document 1: Japanese Un-Examined Patent Application Publication No. H-9-316567    Patent Document 2: Japanese Un-Examined Patent Application Publication No. 2000-144282    Patent Document 3: Japanese Un-Examined Patent Application Publication No. H7-335685    Patent Document 4 Japanese Un-Examined Patent Application Publication No. 2001-345342